The present invention relates to an improved method of producing and delivering of a semi-solid thixotropic metal slurry for use in metal forming processes.
Present commercial metal forming processes employ either fully liquid metals or fully solid metals. Metal forming processes such as sand castings, die castings, and the like employ fully liquid metals while processes such as forgings, extrusions, etc., employ fully solid metals. Existing cast methods in which a metal is brought to a liquid state and then poured or forced into a mold have a number of shortcomings. In casting, when the liquid changes to solid, shrinkage of about 5% is encountered which initiates stress generations which results in cracking and casting porosity. In addition, the fully liquid melt is highly erosive to dies and molds and the high temperature of the liquid and its erosive characteristics makes difficult die casting of some high temperature alloys. The foregoing shortcomings can be alleviated by casting a controlled semi-solid mixture in the form of a thixotropic slurry. Traditionally, forming processes did not employ semi-solid metals because in the conventional solidification of the metals, a dendritic network structure forms when the alloy is as little as 20% solid. Such partially solidified metal cannot be deformed homogeneously without cracking or forming segregates.
The metal composition of a thixotropic slurry comprises primary solid discrete particles and a secondary phase. The secondary phase is solid when the metal composition is frozen and is liquid when the metal composition is partially solid and partially liquid. The primary solid particles comprise small degenerate dendrites or nodules which are generally spheroidal in shape. The primary solid particles are made up of a single phase or plurality of phases having an average composition different from the average composition of the surrounding matrix, which matrix can itself comprise primary and secondary phases upon further solidification. The primary solids obtained in the composition differ from normal dendritic structures in that they comprise discrete particles suspended in a liquid matrix. Normally, solidified alloys have branched dendrites separated from each other in the early stages of solidification and develop into an interconnected network as the temperature is reduced and the weight fraction solid increases. On the other hand, the structure obtained in thixotropic metal slurries consists of discrete primary particles separated from each other by a liquid matrix even up to solid fractions of 80 weight percent. The primary solids are degenerate dendrites in that they are characterized by smoother surfaces and less branched structures which approach a spherical configuration. The secondary solid which is formed during solidification from the liquid matrix, subsequent to forming the primary solid, contains one or more phases of the type which would be obtained during solidification of the liquid alloy in commercial casting processes. That is, the secondary solid can comprise dendrites, single or multi phase compounds, solid solutions, or mixtures of dendrites, compounds and/or solid solutions.
The known method used to prepare a thixotropic slurry as described above is disclosed in U.S. Pat. Nos. 3,948,650 and 3,902,544. The method comprises raising the temperature of an alloy to a value at which most or all of the alloy is in the liquid state and then agitating or stirring the liquid or semi-solid metal. The temperature of the melt is reduced to increase the solid fraction while agitating or stirring the melt to form discrete degenerate dendrites while avoiding the formation of a dendritic network. It is required that the agitating or stirring produce shear rates sufficient to break up the dendritic network structure traditionally formed during solidification and produce a slurry comprising solid spheroids dispersed in a liquid. As disclosed in the aforesaid patents, the preferred apparatus for agitating or stirring the molten metal slurry consists of a metal rod inserted into a cylindrical tube or chamber containing the solidifying alloy. In order to produce the necessary shear rates sufficient to break up the dendritic network structure when mechanically stirring as disclosed in the aforenoted patents, two parameters are critical. Firstly, in order to produce the necessary shear rates in the region of the stirring rod, the rod must be rotated at speeds in the range of 1,000 rpm. Secondly, since the effective shear rate in the slurry rapidly dissipates in areas radially removed from the stirring rod, there is a critical annular gap size between the rod and cylinder wall containing the metal which must be maintained in order to effect the necessary shear rate throughout the metal slurry. As a result of this procedure, extreme wear and erosion of the stirring rod occurs. Furthermore, as a result of the criticality of the gap maintained between the rod and the cylindrical tube, volumetric throughput is extremely limited. As a result of these disadvantages in the processes of the above-noted patents, the commercial exploitation of producing thixotropic slurries for rheocasting and thixocasting has been extremely limited.
The present invention contemplates an improved method for the preparation and delivery of semi-solid thixotropic metal slurries for use in casting processes which provide a high volume supply of semi-solid slurry.
Accordingly, it is the principal object of the present invention to provide an improved method of vibrating molten metal during solidification so as to produce a highly fluid semi-solid slurry with a degenerate dendritic structure comprising solid spheroids dispersed in liquid.
It is a further object of the present invention to provide a method for producing a thixotropic slurry which is capable of providing high flow rate delivery of the semi-solid slurry.
It is still a further object of this invention to provide an effective economical and commercial process for preparing and delivering semi-solid thixotropic slurries for use in metal forming processes.